Phosphinic esters are valuable synthesis building blocks and can be used, for example, for preparing polymers and plastics in order to obtain low-flammability materials.
Thus, DE 26 52 007 A1 describes low-flammability epoxy resins by incorporation of carboxy-phosphinic acids. U.S. Pat. No. 5,399,428 A1 describes flame-resistant linear polyesters by incorporation of carboxy-phosphinic acids.
DE 25 40 283 A1 describes the addition of phosphines to .alpha.,.beta.-unsaturated carboxylic acids in the presence of aqueous hydrochloric acid and subsequent oxidation.
DE 28 49 003 describes the preparation of phosphorus-containing cyanohydrin derivatives by addition of phosphonous esters to acrolein cyanohydrin derivatives.
Phosphinic esters are obtained by adding phosphonous monoesters to 1-olefins in the presence of peroxide catalysts. However, the yields are only low. The addition of phosphonous monoesters to activated double bonds in the presence of alkoxides as catalyst proceeds better. Suitable unsaturated compounds are .alpha.,.beta.-unsaturated carboxylic esters or carbonitriles, .alpha.,.beta.-unsaturated ketones and alkyl vinyl sulfones and vinyl acetate (Houben-Weyl, Volume 12/1, pp. 258-259).
The phosphonous monoesters themselves are prepared from phosphonous dihalides by reaction with alcohols or by hydrolysis and subsequent esterification.
Functional phosphinic acids are obtained by reacting phosphonous dihalides (dihalophosphines) with activated olefinic compounds such as acrylic or methacrylic acid derivatives and then carrying out hydrolysis (Houben-Weyl, Volume 12/1, p. 230; K. K. Khairullin, T. I. Sobchuk, A. N. Pudovik, Zh. Obshch. Khim. 37, 710 (1967)). Byproducts produced are the halides of the organic acids used in the hydrolysis.
In addition, phosphonous dihalides can also be reacted with alkyl halides in the presence of aluminum chloride (Houben-Weyl, Volume 12/1, p. 232). Phosphinic esters can be prepared from phosphonous dialkyl esters by the Michaelis-Arbuzov reaction. Phosphonous dialkyl esters are in turn prepared from phosphonous dihalides and hydroxyl compounds.
The abovementioned phosphonous dihalides, e.g. methyldichlorophosphine, which can be used as starting materials for other syntheses, have been prepared themselves to date in a complex synthesis from phosphorus trihalides and alkyl halides in the presence of aluminum chloride (Houben-Weyl, Volume 12/1, p. 306). The reaction is highly exothermic and it can be managed industrially only with difficulty. In addition, various byproducts are formed which, as are also some of the abovementioned starting products, are toxic and/or corrosive, that is highly undesirable.
There is therefore a requirement for a process for preparing phosphinic esters, which process can be carried out in a simple manner and in which uniform products are obtained in a high yield. Such a process should also be considerably superior environmentally to those known hitherto.